A deactivated mailbox is usually one of two things:
Billing. If a payment failed, your accounts are paused until billing is updated. Update your payment method from your billing page to reactivate.
Sending limit. Mailboxes are automatically deactivated if they exceed 100 emails per day per mailbox. To stay safe, keep to about 80/day per mailbox (roughly 50 warmup + 30 outreach), which leaves room for replies and tests.
Contact our support and we'll reactivate the affected mailboxes.